Piezoelectric sensor

ABSTRACT

A piezoelectric sensor includes a carrier, a piezoelectric measurement sensing element arranged on the carrier, a covering layer covering the measurement sensing element and an electronic evaluation unit. The measurement sensing element is formed by a piezoelectric layer. The carrier has a first contact layer electrically connected to the piezoelectric layer and the covering layer has a second contact layer electrically connected to the piezoelectric layer. The electronic evaluation unit is able to determine a mechanical loading of the piezoelectric layer by evaluating the difference of electrical potential between the first contact layer and the second contact layer.

RELATED APPLICATIONS

This application claims the benefit of PCT International ApplicationSerial No. PCT/DE01/02362, filed Jun. 29, 2001 which claims the benefitsof German Utility Model Application Serial No. 100 31 793.6, filed Jul.4, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to piezoelectric sensors.

2. Description of the Related Art

A piezoelectric sensor is known from German Patent 28 431 938. In thissensor the charge transfer of a piezoelectric film, which is convertedinto the desired measurement signal via an external electronicevaluation unit, is used as the measurement value. These sensors havethe disadvantage that the length of the paths to the electronicevaluation unit are limited and a further component must be arrangedremotely with the electronic evaluation unit.

Sensors of the above-mentioned type are known in addition as impactsensors having a seismic mass, the seismic mass being pressed againstthe piezoelectric layer as a result of an impact and the accelerationcaused thereby. This pressure on the piezoelectric layer causes, inturn, a charge transfer which can be picked up and evaluated by means ofcontacts on each side of the surface of the piezoelectric layer. Asensor of this type is described by Gevatter in “Handbuch der Mess- undAutomatisierungstechnik”, VDI Verlag 1998.

These sensors with a seismic mass have, in addition to the disadvantagesof the above-mentioned film-type sensors, the disadvantage that theseismic mass must be excited. Furthermore, because of the movableguidance system of the seismic mass, a comparatively complex and costlymechanism must be provided which, because of the moving parts, leads tohigh production costs and, in addition, to a higher risk of failure.Finally, construction of very sensitive sensors is possible only atdisproportionate cost, as measurement is only possible if the seismicmass is excited by the weak signal. For this reason a sensor of thiskind can hardly be used, for example, as a vibration meter.

Hence, those skilled in the art have recognized a need for a sensorwhich can be manufactured at low cost, is easily installed and canmeasure economically and in a simple manner even weak signals, such asvibrations or material deformations, by utilizing the piezoelectriceffect. The invention fulfills these needs and others.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the invention is directed to apiezoelectric sensor including a carrier, a piezoelectric measurementsensing element arranged on the carrier, a covering layer covering themeasurement sensing element and an electronic evaluation unit. Themeasurement sensing element is formed by a piezoelectric layer. Thecarrier has a first contact layer electrically connected to thepiezoelectric layer, the covering layer has a second contact layerelectrically connected to the piezoelectric layer and the electronicevaluation unit is able to determine a mechanical loading of thepiezoelectric layer by evaluating the difference of electrical potentialbetween the first contact layer and the second contact layer.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a sensor according to the invention;

FIG. 2 is a top view of the sensor illustrated in FIG. 1; and

FIG. 3 shows a networking of a plurality of sensors for monitoring anindustrial manufacturing process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein the reference numerals denotelike or corresponding parts throughout the figures, and particularly toFIG. 1, there is shown a piezoelectric sensor formed by a carrier 2 anda film-like piezoelectric layer 1 arranged thereon. The piezoelectriclayer 1 is covered by a covering layer 3, both the covering layer 3 andthe carrier 2 are configured to be electrically conductive to thepiezoelectric layer 1. For this purpose the carrier 2 is provided atleast on its side facing towards the piezoelectric layer 1 with a firstcontact layer 5. The covering layer 3 is provided with a second contactlayer 6. Both layers, the first contact layer 5 and the second contactlayer 6, can be produced on the respective component by means of vacuumevaporation; alternatively thin metal or precious metal foils can bebonded thereto.

In the front area, the covering layer 3 is connected to the carrier 2,the conductive second contact layer 6 is connected to an internalelectronic evaluation unit 4, also arranged on the carrier, by means ofconductive tracks. The first contact layer 5 is connected to theelectronic evaluation unit 4 likewise by means of conductive tracks.This electronic evaluation unit 4 can carry out the complete evaluationof the measurement signal or can take over only a part of the signalprocessing and can transmit an intermediate signal to an external signalprocessing unit 8 (not shown). For this purpose the connecting cable inthe exemplary case illustrated is provided with a standardized interface9 which makes possible simple connection of the sensor to a field bussystem.

For reasons of clarity dimensions are not reproduced to scale in thefigures. In practice the piezoelectric layer 1, like the first contactlayer 5 and the second contact layer 6, will be significantly thinner.In the embodiment shown the sensor is surrounded by a housing 7,illustrated here as a hollow housing. However, the housing 7 ispreferably injection molded around the functional components so that thelatter are enclosed in an air-free manner. The housing 1 can alsoenclose the carrier 2 or can be closed laterally by the latter. In bothcases the housing 7 must be so designed that twisting of thepiezoelectric layer 1 analogously to the vibration or deformation of thecomponent on which the sensor is arranged remains possible.

FIG. 2 shows the sensor illustrated in FIG. 1 in a top view, partiallyin cross-section. In the right-hand area the piezoelectric layer 1 withthe covering layer 3 is arranged in a sandwich structure on the carrier2, the first contact layer 5 and the second contact layer 6 beinginterposed respectively between the above-mentioned elements. Theinternal electronic evaluation unit is represented schematically hereand consists of a miniature integrated circuit having the usualelectronic components for signal processing, which integrated circuitcan be formed by the left-hand part of the carrier 2 or can be placedthereon. The covering layer 3, for example a thin silver foil, isconnected in the area facing towards the electronic evaluation unit 4 toa contact element which in turn is connected to the electronicevaluation unit 4 via conventional conductive tracks. The bonding to thefirst contact layer 5 located below is effected in a similar manner.

In one embodiment of the invention the evaluation of the measurementsignal is not carried out exclusively via the internal electronicevaluation unit 4. In this case the interface 9 is used for connectionto an external electronic evaluation unit 8 which takes over furthersignal processing. By this means triggering and initializing of thesensor can be carried out. In particular in the case of very weaksignals which can occur, for example, when the sensor is used as avibration sensor for detecting disturbance signals, the cost requiredfor miniaturization can be reduced by means of an external electronicevaluation unit 8.

FIG. 3 shows an exemplary application of the invention in which defectsand damage to pumps can be detected by monitoring the solid-borne soundin the pump housing. In this case normal vibrations are manifested bypump noises which, in case of damage, for example, damage to a pumpwheel, are changed in frequency or amplitude. For this purpose thesensor according to the invention first measures the “normal” pump noiseand records it as operating noise. In case of changes to this noise thechange can be filtered out by frequency analysis and interpreted as adefect or as a normal change caused by operating conditions.

In the application illustrated in FIG. 3 the individual sensors areinterconnected via a field bus which makes possible connection of theinterfaces 9 to the external electronic evaluation unit 8. For thispurpose a common evaluation and control unit is connected to thesensors, a display and operating element being interposed. If defectsare detected the required measures can be taken by means of conventionalcontrol systems.

The sensor together with the evaluation electronics is configured tohave the thickness of a film, the piezoelectric layer having a thicknessof less than 1 mm and the evaluation electronics together with themeasurement sensing element being arranged on the film-like carrierwhich is manufactured from an elastic material which damps vibrations toonly a slight degree.

Through the configuration of the sensor according to the invention theseismic mass is dispensed with and a thin, piezoelectric layer arrangedon the elastically deformable carrier is used as the measurement sensingelement, the deformations of which carrier it converts into ameasurement signal. The evaluation electronics are arranged on thecarrier so that transmission of the weak piezoelectric measurementsignal from the film to the evaluation electronic unit over a relativelylong distance is unnecessary. In this way the cost of manufacturing andinstalling the sensor can be considerably reduced and the sensor can beprovided for the first time as a piezoelectric sensor for entirely newapplications.

The assemblies of the new type of sensor are the flexible carrier andthe covering layer, together with the piezoelectric layer arrangedtherebetween. The covering layer and the carrier are so configured thatthe charge transfer within the piezoelectric layer as a result of adeformation can be picked up by the carrier and covering layer. Thedeformation of the piezoelectric layer can be both a twisting about anyaxis and a pressure acting in a vertical direction on the layer itself.

All the components of the sensor are sufficiently flexible for thesensor, which can be bonded to a surface, to be able to convertvibrations present in the component carrying the sensor as a result ofsolid-borne sound, into vibrations of the piezoelectric layer. This inturn produces a charge layer which changes over time as a function ofthe amplitude and duration of the vibration, which charge layer can bepicked up via the carrier and the covering layer and converted into thedesired signal by the evaluation electronics.

A sensor constructed in this manner can be used in a multiplicity ofapplications and, because it can be manufactured at especially low cost,can often be integrated as an additional link in a control chain. Forexample, in a preferred embodiment of the sensor, the sensor can be usedin the context of triggering an airbag of a motor vehicle, which airbagsare at present triggered almost exclusively by impact sensors. Inparticular, in accidents in which the vehicle is first subjected to animpact but has not yet suffered the accident, for example when collidingwith a safety barrier and subsequently skidding, the airbag isfrequently triggered by the first impact and, because it collapses againafter a few fractions of a second, is no longer available during theactual crash of the vehicle. In this situation the sensor according tothe invention can be used in addition to the electronic system and, forexample, can be bonded to the inside of a body panel of the vehicle. Thesensitivity of the sensor can be so adjusted via the evaluationelectronics that a relatively large deformation, in addition to theimpact, must be necessary to trigger the airbag.

The piezoelectric layer of the sensor is only a few μm thick and ispreferably manufactured from a film. Piezoelectric film of this kind isobtainable by the meter, the price for 1 m² of film being approximatelythe same as the price of the piezoelectric sensors with seismic massused hitherto. A large number of sensors can be manufactured from 1 m²of film, so that, bearing in mind the fact that the remaining componentsof the sensor involve no significant costs, the sensor according to theinvention can be manufactured approximately 10 to 50 time more cheaplythan the sensors known and used hitherto. This makes its use in manyapplications economic for the first time. Multiple monitoring by the useof a plurality of sensors also now becomes possible and economic.

Through the use of the piezoelectric film in almost any desired shape animpact sensor can be constructed, for example, as protection againstpinching. For example, in the case of a roll-up door, a lower rubber lipcan be provided with the measurement sensing element, the carrier beingformed by an outer rubber layer and the covering layer by an innerrubber layer. When the rubber lip makes contact at any point the sensorwill emit a signal which can be used to switch off the door mechanismpermanently or temporarily in case of jamming. This can also be used fordoors of a public transport vehicle or for electrically-actuatedwindows.

Further exemplary applications of the sensor according to the inventionare the detection of vibrations as a result of machine damage, such ascan occur in pump housings, in shaft bearings or in rail vehicles. Forexample, the fracture of a wheel rim which would endanger trainoperation can be detected by the additional vibrations occurring as aresult of the fracture and a signal can be emitted, or a controlintervention made, in good time before derailment of the train. In thecase of a pump housing, which must normally tolerate a uniform vibrationas a result of splashing of the liquid to be pumped or as a result ofbearing noise, the deviation from the normal frequencies or amplitudesin case of bearing damage can be detected and action can be taken beforemore serious damage occurs. Finally, the sensor can also be used for theconstruction of alarm systems or movement detectors, for example whendisplay windows or motor vehicles are to be protected from maliciousdamage by scratching.

The carrier is preferably manufactured from a flexible material, forexample a flexible plastics material. The first contact layer, which canbe formed, for example, by a vacuum-evaporated coating of silver, canthen be applied to this material. Bonding on of a metal foil or a foilmade of another conductive material is also possible.

In a preferred embodiment of the invention the carrier is divided intotwo areas, the actual sensor being arranged in a sensor area while atleast a part of the evaluation electronics is arranged in an adjacentarea. In the sensor area the first contact layer is applied, whileleaving free an edge portion, to which contact layer the thinpiezoelectric film of the same size can then be bonded. The coveringlayer, which carries the second contact layer in the area of thepiezoelectric film, is applied in turn to this piezoelectric film. Thissecond contact layer can also be applied to the covering layer by vacuumevaporation or can be bonded on as a foil. The first contact layer andthe second contact layer are then connected to the evaluationelectronics, and have in particular a cable connection to correspondingcontacts or are so configured that they have prolongations extending inthe manner of conductive tracks in the direction of the evaluationelectronics and connected to the corresponding contacts.

In an alternative embodiment of the invention the contact layer can alsobe arranged inside or below the carrier or the covering layer, in whichcase the electrical contact to the piezoelectric layer is made viathrough-connections.

Preferred thicknesses of the piezoelectric layer are less than 1 mm, inparticular a few μm, in particular less than 20 μm, thicknesses evenbelow 10 μm being possible. In a concrete embodiment of the sensor as avibration meter a thickness of the piezoelectric layer of, for example,6 μm is used. This piezoelectric film is joined to the carrier and thecovering layer with inclusion of the first contact layer and the secondcontact layer, the connection being effected in particular by bondingduring the manufacturing process.

The carrier can be manufactured from an electrically non-conductivematerial, although it is also possible for the carrier itself to beconductive. In this case it is no longer necessary to apply a separatefirst contact layer and it is sufficient if the carrier itself forms thefirst contact layer. The same applies in an analogous manner to thecovering layer which, of course, can also be electrically conductive.The carrier and the covering layer can have a thickness of less than 1mm, a material thickness between 120 μm and 160 μm being especiallypreferred. In the case of a concrete embodiment the first contact layerand the second contact layer have a thickness of 5 to 50 μm, so that atotal thickness of the sensor in the area of the measurement sensingelement of less than 300 μm (without housing) results.

By dispensing with the seismic mass the sensor can therefore be keptvery thin, enabling it to be used where it has not hitherto beenpossible to use piezoelectric sensors or sensors of any kind. At thesame time the sensor has good resistance to pressure. For this reason itcan be used as a washer in a screw connection, the pressure sensingelement being clamped by the screw insertion force and the evaluationelectronics being arranged beside the sensor. By means of this sensorthe screw insertion force and an impact load on the screw can bemonitored and, for example, slackening of the screw or inadmissibletightening can be detected at an early stage.

The possibility of using the sensor as a pressure-loaded impact sensorin a screw connection makes it possible to measure and control thesealing force of a packaging machine, in which the two halves of thetool are pressed against one another in the context of vacuum packaging.In this case the sealing force can be measured via measurement of theretention force of the clamping screws and regulated if necessary.Finally, an internal pressure in a chamber, for example, can also bemeasured by means of a sensor of this kind.

In the case of an extruder, for example, the internal pressure in thefront area of the extruder can be measured by means of theabove-mentioned principle via the retention force of the screws by whichthe orifice cap is fixed to the extruder housing, without the need tointroduce a pressure measuring device into the chamber by means ofcomplex and expensive constructional measures. In the case of aninternal combustion engine a sensor could be arranged in the area of thevalve-actuating cam, which sensor measures the closing pressure of thevalve and therefore the pressure in the combustion chamber. By comparingthe internal pressures in the cylinders a defect can thereby be quicklyand reliably detected without major complexity or cost.

Through the omission of the seismic mass a very sensitive sensor isproduced at low manufacturing cost. A sensor manufactured according tothe above-mentioned principle can, for example, if bonded to atable-top, detect whether or not speaking is taking place in a room. Itis therefore so sensitive that it can convert the sound wavestransmitted to the table-top and further transmitted therein assolid-borne sound, into a measurement signal. A conventionalpiezoelectric sensor with seismic mass would require a very costlymethod of mounting this mass.

The first and second contact layers can also be subdivided into segmentsto construct a still more sensitive sensor, in which case the evaluationelectronics should pick up the potential difference between each twoopposed segments of the first and second contact layers. In this way,not only can spatial information on the charge transfer within thepiezoelectric layer be obtained but the risk of failure of the sensorcan be reduced, since a plurality of contacts are present and thefailure of one pair of segments will not cause the failure of thesensor. In particular in the case of self-calibration of the sensor, thesensor is so adjusted by the subsequent calibration process in case offailure of one pair of sensors, for example as a result of a brokencable, that adjacent segments can take over the function of the failedpart.

The sensor is preferably integrated together with the evaluationelectronics in a housing, which housing can be produced especiallyeasily by molding the functional components into a plastics block. Thecarrier can be installed on a further carrier plate which can be mountedat the actual point of application, for example a vehicle body panel ora pump housing. To transmit the vibrations of the housing, which, it inthe case of a vibration meter, are to be measured, either the carrier orthe additional interposed carrier element can be bonded to the housingto be monitored.

Instead of a rigid housing the housing can also be formed by a filmwhich encloses the remaining functional parts of the sensor itself. Forthis purpose, in a manner similar to the technique known from vacuumpackaging, a lower and an upper film can, for example, be provided, thesensor being arranged either completely or partially between thesefilms. After evacuation of the cavity these films are either joined bymeans of a continuous peripheral seam, for example by welding or bymeans of a bonded seam. The sensor manufactured in this way can, forexample, then be bonded to the component to be monitored.

The evaluation electronics preferably include a programmable amplifierwhich makes it possible to tune the evaluation electronics to thevibration to be measured in relation to the background signal. Theevaluation electronics can be completely arranged on the carrier,although it is also possible for the evaluation electronics to bearranged only partially on the carrier and for an external part of theevaluation electronics to be housed in a separate housing. The latter isthen connected to the sensor itself via a digital cable or another cablemeeting a conventional bus standard. Both the evaluation electronics onthe carrier and external evaluation electronics preferably have aninterface for a standard bus system. In this way the sensor according tothe invention can be easily and simply integrated into a control system,or example in the context of a monitored manufacturing process.

The evaluation electronics preferably include a signal processor and aprogram memory in which is stored software which controls the sensor andevaluates the measurement or the measured value. By means of frequencyanalysis of the time behavior of the measured potential difference thesoftware detects an unusual frequency change and in this case emits asignal. In this way the regular pump noise caused by splashing andbearing noise in the case of pump monitoring cannot trigger a signal,whereas damage to a bearing shell and an unusual frequency of thesolid-borne sound caused thereby triggers a control signal or even aspeed reduction or unloading of the pump.

To be able to perform this function the software must be able todistinguish the usual operating noise from unusual noises. For thispurpose a self-calibrating routine is preferably provided which is runfrom time to time and at the start of operation. For this purpose thesoftware records the change over time of the solid-borne sound during apreset period, this period being initially interpreted as the normalcase. During subsequent measurements the measurement signal is then ineach case compared to the signal measured in the reference period and anunusual deviation is interpreted as a defect or damage. In this case analarm signal can be emitted or an automatic intervention can be made inthe control system of the device monitored. To avoid false alarms orother malfunctions a tolerance limit can be preset which must beexceeded for the fault signal to be emitted. This tolerance limit canlikewise be adjustable.

By a particular configuration of the evaluation electronics the sensor,which operates in principle as a dynamic sensor, can also be used as astatic sensor at least for a certain time period. For this purpose theelectronics include an integrator which integrates the measured signaland thereby determines the actual state even without a change over timeof the signal. By the use of digital evaluation electronics the value ofthe magnitude measured can also be permanently determined.

By means of an additional pressure mass the sensor according to theinvention can also be used as a pressure sensor or an accelerometer.This pressure mass can be formed, for example, by a flat pressure platewhich is mounted in a perpendicularly displaceable manner with respectto the sensor. The sensor can be arranged between a surface of acomponent to be monitored and the pressure plate, whereby anacceleration in both directions can be measured by means of anadditional spring force acting between the pressure plate and thesensor. This additional compressive force can be formed by one or moresprings or by a resilient intermediate layer which is arranged, forexample, between the sensor and the pressure plate and causes acontinuous loading of the sensor.

When the pressure plate is accelerated either a force in the directionof the measurement sensing element or an unloading of the measurementsensing element is obtained, causing a charge transfer within thepiezoelectric layer. The pressure plate can cover the entire sensor orcan act only on a partial area thereof. In the case of a possibleembodiment of the sensor configured as an accelerometer the housing canbe screwed to a component to be monitored and the pressure plate canhave sufficiently large through-holes for it to be fixable to the sensorby means of the screws used. In this case, of course, the housing mustbe so configured that transfer of the compressive forces from thepressure plate to the piezoelectric measurement sensing element ispossible.

Although, with this extension of the measurement sensing element, aseismic mass is again incorporated the advantageous properties of thesensor according to the invention can nevertheless now be combined withthe additional performance feature of acceleration measurement. Forexample, a very sensitive, low-cost sensor can be constructed accordingto the inventive principle which can also detect an acceleration withouttwisting or other deformation of a component. By contrast, although aconventional sensor with seismic mass could measure acceleration, toachieve the same sensitivity in measuring solid-borne sound it wouldneed to be constructed in a very much more complex and expensive manner,since vibrations can only be measured if the seismic mass is excited. Bymeans of the invention this detour via the utilization of massexcitation can be dispensed with.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. A piezoelectric sensor comprising a carrier, apiezoelectric measurement sensing element arranged on the carrier, acovering layer covering the measurement sensing element and anelectronic evaluation unit, the measurement sensing element being formedby a piezoelectric layer, the carrier having a first contact layerelectrically connected to the piezoelectric layer, the covering layerhaving a second contact layer electrically connected to thepiezoelectric layer and the electronic evaluation unit being able todetermine a mechanical loading of the piezoelectric layer by evaluatingthe difference of electric potential between the first contact layer andthe second contact layer, wherein the sensor together with theelectronic evaluation unit is configured to have the thickness of afilm, the piezoelectric layer having a thickness of less than 1 mm andthe electronic evaluation unit being arranged beside the measurementsensing element on the film-like carrier which is manufactured from anelastic material which damps vibrations to only a slight degree.
 2. Thepiezoelectric sensor of claim 1 wherein the carrier and thepiezoelectric layer are joined together in a planarly adhering manner.3. The piezoelectric sensor of claim 2 wherein the thickness of thepiezoelectric layer is less than 10 μm.
 4. The piezoelectric sensor ofclaim 1 wherein the carrier has a carrier layer made of an electricallynon-conductive material which is provided with the first contact layerand on which, beside the piezoelectric layer, the electronic evaluationunit is arranged.
 5. The piezoelectric sensor of claim 1 wherein next tothe electronic evaluation unit a planar sensor area is built up in asandwich structure, the carrier being provided in the sensor area with afirst contact layer, the piezoelectric layer being arranged on the firstcontact layer and electrically connected thereto and the covering layerhaving the conductive second contact layer which faces towards thepiezoelectric layer being arranged on the latter.
 6. The piezoelectricsensor of claim 1 wherein the carrier and/or the covering layer is/areformed by a flexible film the thickness of which is less than 200 μm. 7.The piezoelectric sensor of claim 1 wherein the thickness of the firstcontact layer and the thickness of the second contact layer are in eachcase less than 70 μm.
 8. The piezoelectric sensor of claim 1 wherein ithas a housing containing the other components.
 9. The piezoelectricsensor of claim 1 wherein the housing is manufactured by molding ofplastics material around the sensor.
 10. The piezoelectric sensor ofclaim 1 wherein the housing is formed by an upper and a lower film whichare joined together by a connecting seam surrounding the remainingcomponents of the sensor.
 11. The piezoelectric sensor of claim 1wherein the first contact layer and the second contact layer are formedby thin foils of a metallic material.
 12. The piezoelectric sensor ofclaim 11 wherein the foils of the first contact layer and the secondcontact layer have in each case at least one cable-like prolongationwhich is connected to the electronic evaluation unit for bonding. 13.The piezoelectric sensor of claim 1 wherein the first contact layer andthe second contact layer are subdivided into segments by means ofelectrically insulating gaps and the electronic evaluation unit isconnected to each of the segments in such a way that it can determinethe potential difference between a pair of segments of the first contactlayer and of the second contact layer.
 14. The piezoelectric sensor ofclaim 1 wherein the electronic evaluation unit includes a programmableamplifier.
 15. The piezoelectric sensor of claim 1 wherein theelectronic evaluation unit has an interface for connection to a fieldbus, in particular a CAN bus.
 16. The piezoelectric sensor of claim 1wherein the electronic evaluation unit includes a signal processor and aprogram memory with software stored therein, the software being able todetect an unusual frequency change by means of frequency analysis of themeasured potential difference for measurement of solid-borne sound. 17.The piezoelectric sensor of claim 16 wherein the software includes acalibrating routine which after a preset period identifies the changeover time of the solid-borne sound as the normal case and uses thelatter as the basis for subsequent control measurements, the measuredsignal being compared to that of the normal case and the software beingable to emit a signal in case of deviations above a preset tolerancelimit.
 18. The piezoelectric sensor of claim 1 wherein it has a secondexternal electronic evaluation unit which is connected to the electronicevaluation unit via a digital connection and has an interface forconnection to a field bus system.
 19. The piezoelectric sensor of claim1 wherein it has a pressure plate arranged parallel to the piezoelectriclayer, which pressure plate is guided movably in a directionperpendicular to the piezoelectric layer.
 20. The piezoelectric sensorof claim 19 wherein a preloaded resilient element is arranged betweenthe piezoelectric layer and the pressure plate.